The present invention relates to metal-cleaning compositions which are inhibited against corrosion. Further, the invention relates to a method of using said compositions to chemically clean oxide scale from metal surfaces.
In almost any type of metal equipment in which water is evaporated or heat transfer occurs, insoluble salts deposit on the surfaces to form scale. Usually, the deposits consist of calcium and magnesium salts and result from the use of hard water, although sometimes the scale is based on elements other than calcium or magnesium. Common types of scale which deposit on metal surfaces include calcium sulfate, calcium carbonate, complex calcium phosphate, and calcium oxylate. Additionally, high iron content scales, such as magnetite or hematite, are also encountered. Scale or salt deposits reduce the heat transfer efficiency of the equipment in which they form. Therefore, it is desirable to remove scale and salt deposits from the metal surfaces of heat transfer equipment systems.
The art of cleaning steam boilers and associated piping and vessels has progressed from the use of highly acidic solutions for the dissolution of hard water scale and rust or magnetite to the current use of materials which complex iron or calcium and magnesium as well. Solutions of complexing agents do not rely on mineral acids to accomplish scale loosening or dissolution. In fact, many currently-used cleaning solutions are often made alkaline by the addition of ammonia. For example, see U.S. Pat. No. 3,413,160.
The cleaning solutions which are employed to remove the scale and rust from industrial process equipment, such as boilers and heat exchangers, typically are corrosive to the metal components of the equipment. Thus, the cleaning solutions usually contain corrosion inhibitors. Unfortunately, many corrosion inhibiting compositions are effective only at a specific pH or over a narrow pH range. Additionally, many corrosion inhibitor compositions are relatively insoluble in the cleaning solutions, and a solubilizing alcohol or a surfactant is required. For example, U.S. Pat. No. 4,071,746 discloses an "acid corrosion inhibiting composition" comprising a substituted benzyl pyridinium compound and an oxyalkylated surfactant or an alcohol.
In view of the deficiencies in the prior art compositions, it would be desirable to have a corrosion inhibiting composition which is soluble in effective amounts in concentrated cleaning solutions. Further, it would be desirable to have a corrosion inhibiting composition which could inhibit a large number of corrosive cleaning solutions over a wide pH range.